Whereas the Jews went to the synagogues (or the Temple if they could make the trip) on their last day of the week (Fri night to Sat night on their calendar), the Christians had to determine a time of corporate worship.  They decided on the 1st day of the week in order to celebrate the day of Christ's resurrection.  So in a sense the church changed the day they met for corporate worship. 

 

Some teach that Sunday now represents a "Christian" Sabbath.  I don't have a big problem with that teaching although it is not my belief.  The reality of a Sabbath rest for the Christian is found only in Christ.  It is not a specific day of not working but an ever-present reality that in Christ we rest from our labors trying to gain favor with God.  God has provided His Son, Jesus Christ, as "the way, and the truth, and the life - no one comes to the Father except through Him" (John 14:6, NIV).  It is "by grace you have been saved through faith ... not by works so that no one can boast" (Ephesians 2:8-9, NIV). 

There is a passage in Hebrews that speaks to this directly to the issue of the Sabbath rest for the Christian:

 

 1Therefore, since the promise of entering his rest still stands, let us be careful that none of you be found to have fallen short of it. 2 For we also have had the gospel preached to us, just as they did; but the message they heard was of no value to them, because those who heard did not combine it with faith. 3 Now we who have believed enter that rest, just as God has said, "So I declared on oath in my anger, 'They shall never enter my rest.' " And yet his work has been finished since the creation of the world. 4 For somewhere he has spoken about the seventh day in these words: "And on the seventh day God rested from all his work." 5 And again in the passage above he says, "They shall never enter my rest."  6 It still remains that some will enter that rest, and those who formerly had the gospel preached to them did not go in, because of their disobedience. 7 Therefore God again set a certain day, calling it Today, when a long time later he spoke through David, as was said before:  "Today, if you hear his voice, do not harden your hearts." 8 For if Joshua had given them rest, God would not have spoken later about another day. 9 There remains, then, a Sabbath-rest for the people of God; 10 for anyone who enters God's rest also rests from his own work, just as God did from his. 11 Let us, therefore, make every effort to enter that rest, so that no one will fall by following their example of disobedience. (Hebrews 4:1-11, NIV)

So observing the last day of the week as a "Sabbath" of rest was only a foreshadow of the work of Christ to provide the ultimate rest for those who believe the gospel and put their faith in Him.  In the New Covenant, there is no command to observe a Sabbath day.